How to update text?

[dov]

I'm creating a text node which works fine and I can display text. However, I can't afterwards change the text in the text node.

Here's what I do:

  // In constructor

  // Currently create a fixed font
  string font_filename = "DejaVuSans-Bold.vsgb";
  m_Font = vsg::read_cast<vsg::Font>(font_filename);
  if (!m_Font)
    throw std::runtime_error("Could not load font file " + font_filename);
  m_Font->createFontImages();

  auto layout = vsg::StandardLayout::create();
  layout->horizontalAlignment = vsg::StandardLayout::CENTER_ALIGNMENT;
  layout->horizontal = {0.03,0.0,0.0};
  layout->vertical = {0.0,-0.03,0.0};
  layout->verticalAlignment = vsg::StandardLayout::CENTER_ALIGNMENT;
  layout->position = vsg::vec3(0.0f, -0.015f, 0.0f);
  layout->color = vsg::vec4(0.0, 0.0, 0.0, 1.0);
  layout->billboard = false;
  m_TextLayout = layout;

  auto text = vsg::Text::create();
  text->technique = vsg::GpuLayoutTechnique::create();
  text->font = m_Font;
  text->layout = layout;
  text->text = vsg::stringValue::create();
  text->text->cast<vsg::stringValue>()->value() = "Hello world!";
  text->setup(0, m_Options);
  m_TextNode = text;

So far so good, and if I add m_TextNode to my scene graph, I get hello world.

But when I later try to change the text through the following method

void MyView:::setText(const std::string& text)
{
  auto textNode = vsg::cast<vsg::Text>(m_TextNode);
  textNode->text->cast<vsg::stringValue>()->value() = text; 
  text->text->cast<vsg::stringValue>()->dirty(); // Do I need this?
  textNode->setup(0, m_Options);
}

:
view->setText("Hello again!");

However my setText() call doesn't change the text, and I still get my original "Hello world!" string, but distorted.

What am I missing?

Should create a minimal reproducible example?

[robertosfield]

vsgtext example has dynamic text so have a look at it.

[robertosfield]

Have a look at the other recent threads about dynamic data. If you change the dynamic status of data after compile that would normally register dynamic data you'll need to register it with Transfer task yourself l.

…

On Mon, 29 Sept 2025, 22:03 DovXjet, ***@***.***> wrote: I just had a look at vsgtext.cpp and it appears to me that I do exactly the same thing as for all the dynamic_* variables! However the string displayed is never updated and I'm left with i stretched version of the original string. It seems like the conversion of the text into a mesh/sdf never occurs. How can I debug this? If I can't figure it out, I'll work around it by destroying and rebuilding the text node. — Reply to this email directly, view it on GitHub <#1582 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAKEGUEMWO2AUJAOE7DRCJT3VGNCXAVCNFSM6AAAAACHZE4DRGVHI2DSMVQWIX3LMV43URDJONRXK43TNFXW4Q3PNVWWK3TUHMYTINJUGYYTIMY> . You are receiving this because you commented.Message ID: ***@***.*** com>

[dov]

I finally found a method of modifying the text, which is to recreate text->technique after changing the text! The following code shows the issue:

  auto layout = vsg::StandardLayout::create();
  layout->horizontalAlignment = vsg::StandardLayout::CENTER_ALIGNMENT;
  layout->horizontal = {0.1,0.0,0.0};
  layout->vertical = {0.0,-0.03,0.0};
  layout->verticalAlignment = vsg::StandardLayout::CENTER_ALIGNMENT;
  layout->position = vsg::vec3(0.0f, -0.015f, 0.0f);
  layout->color = vsg::vec4(0.0, 0.0, 0.0, 1.0);
  layout->billboard = false;

  auto text = vsg::Text::create();
  text->technique = vsg::GpuLayoutTechnique::create();
  text->font = m_Font;
  text->layout = layout;
  text->text = vsg::stringValue::create("Initial");
  text->setup(0, m_Options); 

  // Modify the text  
  text->technique = vsg::GpuLayoutTechnique::create(); // Without this the text won't change!
  text->text = vsg::stringValue::create("Modified");
  text->setup(0, m_Options);

  mySceneGraph->addChild(text);

So is this on purpose, or is there some other way to trigger the "technique" to rebuild the texture?

[robertosfield]

There is no reason to rebuild the texture or any other members save for the string data being passed to the GPU.

At a guess try creating a stringValue object that is big enough for all your needs, set it's properties.dataVariance to DYNAMIC then just assign the string you want and fill in the bits of the string you don't need to with spaces or 0.

[dov]

Thanks! You were right (of course :-) ). All that I was missing was:

textNode->text->cast<vsg::stringValue>()->properties.dataVariance = vsg::DYNAMIC_DATA;

and I can then update the text by doing:

  textNode->text->cast<vsg::stringValue>()->value() = newText;
  textNode->setup(0, m_Options);

Note that this is not done in the vsgExample vsgtext.cpp, which still works for whatever reason. But it is probably a good idea to add this property modification to the example to show that it is typically needed.

Here's a screenshot of my use case for the text modification, an interactive draggable caliper.

Thanks!